When and how should I start with marketing.

I have a startup I'm working on(yes, sole founder, yes I know it's going to be tough) which I estimate would be ready for alpha users in about 4 months time.

I have a email register for those interested on the site at the moment. Should I focus on marking only when I have an actual working product?

Also, what would be the best methods to market my product when it's ready?